Electron lenses



Aug. 23, 1960 M. E. HAlNE ETAL ELECTRON LENSES Filed Sept. 11, 1958 INVENTORS MICHAEL EDWARD HAINE 22 24 222 224 :2222 v v 2 2 2 2 2 v J 3 222 HN .H W .H wwvgwwwwmwum w United States Patent O ELECTRON LENSES Michael Edward Haine, Sulhamstead, near Reading, and

Thomas Mulvey, Caversham, Reading, England, as-

signors to Metropolitan-Vickers Electrical Company This invention relates to electron lenses and has an important application in X-ray analysing apparatus of the kind described in co-pending application Serial No. 760,472, in which analysis of elements is carried out by bombarding the surface of a specimen with a beam of electrons and examining the wavelengths of the X-rays thereby produced.

In order to be able satisfactorily to examine a small area of the surface of the specimen it is desirable that the electron beam shall be of small cross-section and also that there shall be a large electron current in order to obtain a satisfactory X-ray emission.

It can also be shown that the current I is determined by the relationship where C is the spherical aberration coefficient of the lens, is the current density per unit solid angle from the cathode of the electron gun. Hence a small spherical aberration is necessary for a large current and in electron lens arrangements, as hitherto employed, this has usually been achieved by locating the specimen in a strong magnetic field between the pole pieces. However, with X- ray analysing apparatus of the kind above referred to it is undesirable that the specimen shall be located in a magnetic field since this precludes an examination of form-magnetic specimens owing to the manner in which they would upset the lens field and furthermore, with electron lenses, as hitherto used, for example in electron microscopes, it is difficult to arrange for the X-rays to pass out of the apparatus as the specimen is located within the magnetic structure and it is also diflicult to make any provision for examining the specimen optically.

The main object of the invention is to provide an improved electron lens arrangement in which the difficulties above referred to are substantially avoided or minimised.

According to the present invention an electron lens arrangement comprises an annular magnet assembly adapted to encircle the beam path concentrically therewith and com-prising axially displaced annular poles of which one pole is formed by the edge of a small aperture in a plate extending transversely across the electron beam path and with the aperture centrally located for the beam and the other pole is formed by a cylindrical plate of large internal diameter relative to the working distance of the specimen and concentric with the beam path and spaced axially from said lateral plate together with means for positioning the specimen in the electron beam path in a region beyond said apertured plate and outside the magnetic structure and in which there is a negligible magnetic field.

The term working distance as used implies the distance between the last working surface of the lens and the focal point at which the specimen is located.

Preferably the working distance of the specimen beyond the structure is large to make it easy to extract the X-ray beam. It will be appreciated that with such an arrangement in which the specimen is located outside the magnetic structure it is possible to extract the X-rays without undue ditficulty as it is not necessary to provide an X-ray passage through the structure and this result is obtained without appreciably increasing the spherical aberration since the diameter of the cylindhical pole is larger.

Furthermore, since the specimen is outside the magnetic field and the transverse apertured plate acts, to a large extent, as a screen between the specimen and the electron lens, it cannot appreciably distort the electron lens field and it is possible to examine specimens of magnetic material, e.g. iron.

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings, in which:

Fig. l is a cross sectional view showing diagrammatically a known form of electron lens for purposes of comparison, whilst Fig. 2 shows an improved electron lens constructed in accordance with the invention, and

fig. 3 shows one embodiment of the invention.

In Fig. 1 the lens shown comprises an annular magnet winding 1 surrounded by a yoke '2, one pole piece is formed by the internal edge 3 of a plate 4 whilst the other pole piece 5 is formed by an internal lip on the end of a cylindrical wall 6; the specimen 7 in such arrangements is located substantially between the pole pieces 3 and 5 and in a region of strong magnetic field to reduce spherical aberration. With this construction if the target is bombarded by electrons projected upwardly along the axis .13 it is difiicult to extract the X-rays.

Fig. 2 shows an improved construction in accordance with the invention in which like parts have been given similar references with a prime In this case, however, it will be seen that the transverse plate 4 is formed with a small central aperture 8 bordered by the annular pole pieces 3' and that this pole is of reduced section. Furthermore, the gap between the pole pieces extends annularly between the pole pieces 5' and the lower face of the transverse plate 4' and the internal diameter of the cylindrical plate 6' is large compared with that of the arrangement of Fig, 1.

It has been found that such an arrangement enables a high current density to be obtained whilst at the same time the specimen 7 is in a negligible magnetic field since it is screened by the plate 4' from the electron lens system within the cylindrical plate 6'. Moreover, the arrangement is highly applicable to an X-ray analyser since the X-rays 14 can be derived from outside the plate 4' and furthermore optical apparatus can be adapted to examine the specimen optically.

Fig. 3 shows an arrangement in which the specimen 7 is mounted on a rotatable carrier 9 mounted on a spindle 10 extending through the cylindrical casing wall 11 of the apparatus.

When the specimen 7 is bombarded by electrons projected upwardly along the axis 13 the X-rays 14 emanating from the specimen are projected through an aperture 12 in the wall 11 and it will be appreciated that this is conveniently placed and the aperture is away from the main magnet assembly and difiiculties which would arise in the Fig. 1 arrangement are avoided.

For a further explanation of an example of apparatus embodying the invention reference may' be made to co pending application, Serial No. 760,472.

What we claim is:

In an X-ray type analyser an electron lens arrangement comprising concentric cylindrical plates encircling the electron beam path, an energising winding located in the annular space between said cylindrical plates, at first annular end plate joining corresponding end edges of the cylindrical plates, an edge to the inner cylindrical plate at the opposite end to said annular plate which edge forms a 'firstmagnetic pole, a transverse plate extending across the electronbeam path in spaced relationship with the pole forming the edge of said innercylindrical plate and secured along its periphery to, the correspond ing end of the outer cylindrical plate, said transverse plate having a circular aperture defined by an edge-concentric with the beam path, saidedg'e forming a second magnetic pole face, means for positioning a specimen in the beam path beyond said transverse plate, a cylindrical casing extending axially of the beam path away from said transverse plate and encircling the beam path and specimen and an aperture in said casing adapted to permit X-ra-ys from the specimen to pass out from the casing.

References Cited in the file of this patent UNITED STATES PATENTS 2,440,640 Marton Apr. 27, 1948 10 2,849,619 Eisfeidt Aug. 26, 1958 FOREIGN PATENTS Sweden Feb. 13, '1945 

